1. Field of the Invention
The present invention relates to an air flow metering apparatus for metering suction air flow rate in an internal combustion engine such as a gasoline engine, and, more particularly, relates to an air flow metering apparatus using a heat resistor.
2. Description of the Prior Art
To sufficiently realize a high engine performance and exhaust gas control in cars, a system for controlling engines by detecting suction air flow rate has been put into general practice. With the advance of the system, various kinds of air flow meters have been proposed. For example, a hot-wire air flow meter or sensor is one of the air flow meters now in general use.
In general, an electronic control apparatus for a car is mainly constituted by a micro-computer. It is therefore necessary that the detection signal of such an air flow meter or the like acting as an apparatus for generating an input signal for the control apparatus is taken into the control apparatus in the form of a digital signal after the detection signal has been subject to analog-to-digital conversion. Accordingly, it is desirable to design flow meters for cars so that the output signals thereof can be directly converted to digital signals. To this end, there has been disclosed an air flow meter, so to speak, a pulse-width modulation air flow meter, for example, in Japanese Patent Unexamined Publication No. 62-110123. In the so-called pulse-width modulation air flow meter, the supply of current to a hot wire is made intermittent in synchronism with the rotation of the engine so that the air flow rate can be directly converted into an output pulse-width signal by using the fact that the time required for heating the hot wire to a predetermined temperature is considered to be a predetermined function of air flow rate.
In the proposed air flow meter, however, lots of inherent problems exists.
A first problem is as follows. A predetermined high accuracy will be required for metering suction air flow rate in a car engine, in addition to consideration of the variations in apparatus resulting from the manufacture, the time aging, and the like.
However, the aforementioned prior art lacks consideration for the influence of pulsating suction air and the influence of thermal capacity of the hot-wire probe in the case where the rotation speed of the engine is relatively low and the load upon the engine is relatively heavy. There arises a problem in that a sufficiently high accuracy cannot be achieved.
A second problem is as follows. In the aforementioned prior art, the hot-wire heating period is directly represented by an air flow signal, so that the characteristics of the apparatus variously vary according to the rotation speed of the engine. Accordingly, all the various characteristics must be stored. There arises therefore a problem in that the required memory capacity increases greatly.
A third problem arises in the double points that the output signal expresses an oscillating waveform in the early stage of the stepping change of air flow rate and that the average amount of pulsating air flow rate is measured to be lower than the true amount. This is because the detection signal obtained through intermittent heating is proportional to the average of the heat transmission coefficient, not proportional to the average of the flow rate in the measurement period. In other words, the reason is that the average of the heat transmission coefficient (A+B.sqroot.Q.sub.a), not the average of the flow rate Q.sub.a, is obtained (A and B being constants). Accordingly, errors increase as the amplitude ratio of the pulsating air flow rate increases.